There is a growing requirement for alternate fuels for vehicle propulsion and power generation. These include fuels such as natural gas, bio-diesel, ethanol, butanol, hydrogen and the like. Means of utilizing fuels needs to be accomplished more efficiently and with substantially lower carbon dioxide emissions and other air pollutants such as NOxs.
The gas turbine or Brayton cycle power plant has demonstrated many attractive features which make it a candidate for advanced vehicular propulsion and power generation. Gas turbine engines have the advantage of being highly fuel flexible and fuel tolerant. Additionally, these engines burn fuel at a lower temperature than reciprocating engines so produce substantially less NOx per mass of fuel burned.
The efficiency of gas turbine engines can be improved and engine size can be further reduced by increasing the pressure and temperature developed in the combustor while still remaining well below the temperature threshold of significant NOx production. This can be done using a conventional metallic combustor or a thermal reactor to extract energy from the fuel. As combustor temperature and pressure are raised, new requirements are generated in other components, such as the recuperator and compressor-turbine spools.
In a high efficiency gas turbine engine, the turbine adjacent to the combustor may have a ceramic rotor or it may be an all-ceramic turbine (volute, rotor, rotor shroud). The ceramic rotor is typically attached to a shaft which in turn is usually attached to a compressor which is comprised of a metallic rotor because the compressor blades see much lower temperatures than the turbine blades. The ceramic-to-metal attachment joint represents one of an important feature that, if not designed correctly, can limit the allowable operating temperature of the turbine rotor especially in small turbo-compressor spools such as used in turbo-chargers and microturbines. Most prior art joints are limited to operating temperatures below 800° K. The objective of achieving increased efficiency is pushing the rotor temperatures to levels approaching 1,400° K and, in the future, higher. In the prior art, this joint is typically located close to the turbine rotor, thereby requiring aggressive cooling to maintain the allowable temperature at and around the joint. The steep thermal gradient also creates an area of elevated thermal stress at and around the joint.
There remains a need for a joint design that will allow increased combustor temperatures which, in turn, can improve overall engine efficiency and reduce engine size while maintaining very low levels of NOx production.